1. Field of the Invention
This invention relates to steel sheets for porcelain enameling having good press formability and improved enameling properties such as fishscale resistance, enamel adhesion property, resistance to blistering and pinhole defects and the like, and a method of producing the same.
2. Related Art Statement
Since the steel sheets for porcelain enameling are subjected to severer press forming as represented by the formation of drainboards, bathtubs and the like, it is required to have a fairly deep drawability and satisfy enamel adhesion property (particularly adhesion property in direct enameling at once), firing strain resistance, fishscale resistance and resistance to blistering and pinhole defects.
From the old time, decarburization capped steels are mainly used as a steel sheet for porcelain enameling having a good press formability, but continuously cast Ti-added steels become a main current at the present.
As to the Ti-added steel, Japanese Patent Application Publication No. 42-12348, No. 44-18066 and the like disclose that an excellent press formability is obtained when the C content is not more than 0.005 wt% (hereinafter shown by % simply). Furthermore, Japanese Patent Application Publication No. 45-40655 and Japanese Patent laid open No. 53-131919, No. 56-9357 and the like disclose that such a Ti-added steel also possesses an excellent fishscale resistance.
In the Ti-added steel, Ti is an element forming carbide, nitride or sulfide and is utilized as a precipitate thereof to trap hydrogen in steel causing the fishscale, resulting in the improvement of the fishscale resistance.
However, Japanese Patent laid open No. 61-276958 discloses that the Ti-added steel creates defects due to poor weldability. Further, Japanese Patent laid open No. 60-110845 discloses that the Ti-added steel is poor in the enamel adhesion property and resistance to blistering and pinhole defects as compared with the conventional decarburization capped steel.
Particularly, the above Japanese Patent laid open No. 61-276958 discloses that the blistering defect and the shrinkage created due to the poor weldability are attempted to be improved by adding slight amounts of Se and Te to suppress blowhole defects at a weld zone and shrinkage. However, the Ti-added steel has a problem that the blistering and pinhole defects are easily caused at portions other than the weld zones.
In addition to the Ti-added steel, B-added steels are widely known as a steel sheet for porcelain enameling as disclosed in Japanese Patent Application Publication No. 54-3446 and No. 54-39808. In such a B-added steel, the precipitate such as BN and the like formed by the addition of B is utilized to improve the fishscale resistance, and also there is no problem on the weldability.
In these references, however, box annealing is used as an annealing method, so that the resulting steel sheets are unsuitable for applications requiring a severe press forming because the mechanical properties of the steel sheet, particularly f-value are considerably poor.
For this end, a method of improving the r-value in the B-added steel is disclosed in Japanese Patent Application Publication No. 63-54049. In this case, the heating rate in the annealing is restricted to a particular range of not more than 150.degree. C./hr. Such a heating rate clearly indicates a box annealing. Such an annealing step not only takes considerably many days and runs up the production cost but also is apt to create temperature unevenness in longitudinal and widthwise directions of coils. Particularly, the temperature unevenness in the annealing largely affects a precipitation form of a precipitate effectively preventing the fishscale or a surface segregation exerting on the enamel adhesion property as well as the quality and enameling properties of the coil, and has a drawback that the poor adhesion and fishscale are apt to be caused in use by enameling makers.